[azure-http-specs] Add test for empty open string union

From a recent AI spec:

```tsp
union Something { string; }
```

There are some scenarios where an empty open union could be introduced in a preview API, and later, known values would be added. We shoudl ensure that all emitters support this scenario.

Locate the existing tests for Azure HTTP specs and the emitter coverage for open string unions; the issue does not name a file or test entry point. Run the relevant emitter test suite with `union Something { string; }` and verify that every emitter handles the empty open union before known values are added.

Written by the indexing model from the issue text.